I recently changed a whole bunch of propulsion related items. New prop, cutlass bearing, shaft, coupling and motor mounts. All of this is attached to a single cylinder Deutz 17.5 hp air cooled engine with a Hurth HBW-50 2R transmission.

While we were on the hard I took the opportunity to change the transmission fluid. The stuff that came out was very dark and not super viscous like normal ATF fluid. It had no red tinge either. I put Dexron III in the transmission as all the info I found relating to Hurth transmissions said this is correct. I dont have a manual specification for this transmission, so I had to work with what the web recommended.

A couple things of note:

We never had a single issue with the transmission before all the propulsion changes and fluid change.

We up sized the propeller from 12x11 to 13x12 as per multiple propeller company recommendations. Everything seems to work well, and the engine does not appear to be overworked.

After getting everything finished up, we left the yard. After motoring around and testing things out for a while the transmission began having difficulty shifting. It seems to be at higher rpms where it is difficult to shift. Lower rpms seem to be easier. Rather than being the easy shift that it used to, it requires a lot more force to shift it from forward to neutral. More force than what seems comfortable.

Still learning the ins and outs of all these systems. So first question:

Could the slightly upsized propeller cause any issues like this? From my understanding this wouldn't be the case, but thought it was worth asking.

So I'm led to believe it is the transmission fluid. The Deutz runs slightly hot being air cooled, so I'm wondering if the transmission requires another kind of oil as a result. I've read a couple things about using SAE 30 or 15w-40 oils in transmissions and I'm wondering if this could be the case here as well. Could a hot engine mean not using normal ATF?

Anyways - hope this is enough info to come up with some kind of diagnostic. Waiting to get out of here!